The tissue culture core component will provide three principal services for projects 1-4: (1) It will obtain, passage, and maintain frozen stocks of primary cultures of human foreskin fibroblasts and keratinocytes; (2) It will provide appropriate growth-phase cells, both from primary cultures and immortalized cell lines as necessary for all experiments; (3) It will order, inventory, and quality-control test all media, sera, and growth additives. Concomitant with provision of cells for experiments, the tissue culture core will optimize growth conditions, determine doubling times and lag/log growth periods, and perform cytotoxicity and growth- delay assays using the mutagens at concentrations and schedules indicated by the damage mapping experiments. Because of the commonality of cell types and experimental protocols of projects 1-4, performing tissue culture functions in a core component will improve overall quality and the comparability of the data between projects, and will result in economy of effort and significant cost savings as well.